The invention will be applicable, for example, in motor vehicles for informing the users of the vehicle, in particular the driver thereof.
It is known practice to equip a motor vehicle with a display system, called head-up display. Such a system is placed in the field of view of the motor vehicle driver and displays information concerning the state of the motor vehicle, of the traffic or the like.
In order to avoid disturbing the driving, it is essential for the brightness of the projected image to be adapted to the ambient brightness. It is in particular necessary for the optical power of the projected image to differ greatly in a diurnal journey and in a night-time journey or upon passing into a tunnel. Thus, the brightness of the projected image has to be both visible in all circumstances and not objectionable, in both cases notably for safety reasons. Furthermore, given the significant brightness dynamic range necessary for observing these constraints, the management of the brightness must not be done only by the driver and must therefore be automated, adapting to the current situation of the vehicle.
Thus, it has been proposed to detect the ambient brightness using a brightness sensor installed on the vehicle. This brightness measurement is then used to adapt the brightness of the display. Such a system is for example presented in the patent application KR20120123833. Nevertheless, this solution is unsatisfactory in terms of safety since the information recorded by the sensor is imprecise, and can easily be wrong in case of external disturbances (headlights of an oncoming vehicle, town street lamps, etc.).